1. Technical Field
The present invention relates to a projection zoom lens of a six-group configuration and four-group movement mounted on a projection-type display device or the like, and a projection-type display device having the projection zoom lens mounted thereon. In particular, the present invention relates to a projection zoom lens and a projection-type display device suitable for projecting on a large screen in a movie theater or the like.
2. Related Art
A projector device (projection-type display device) which uses a light valve of a liquid crystal display or a DMD display and has a comparatively long back focal length has come into wide use (see Patent Document 1 (JP-A-8-201690)).
In recent years, a projector that can be applied to a large screen and can project a high-definition image has been used as such a projector device in the movie theater.
A reflective liquid crystal display element or a DMD 3-sheet type is used for the projector device provided for such a use, and a longer back focal length and telecentricity as satisfactory as other projectors are required.
Generally, a value obtained by dividing a projection distance by a screen width is referred to as a throw ratio. The screen size and the distance from the screen to the projection room, that is, the projection distance are various according to movie theaters. Accordingly, in order to project a picture with a proper size for each movie theater, a lens corresponding to each proper throw ratio is necessary, but in consideration of cost it is not a good method to actually prepare all lenses corresponding to proper throw ratios. Thus, this can be addressed by providing a width that can correspond to the throw ratio using a zoom lens.
However, in most of the related-art lenses for projectors, a numerical aperture (hereinafter, also referred to as “Fno.”) is changed during zooming. Generally, the related-art lens for a projector has a slower speed of Fno. on a TELE side than that on a WIDE side. Accordingly, in such a zoom lens, a picture of a movie theater with a high throw ratio gets dark even in movie theaters having the same screen size.
As the zoom ratio of a lens gets higher, the versatility of the lens gets higher. However, in the related-art lens for a projector, as the zoom ratio gets higher, a variable of Fno. gets larger. Accordingly, the lens is not suitable for a movie theater.
To address that the lens is not suitable for a movie theater since Fno. is changed according to a change in the zoom ratio, there has been proposed a lens described in Patent Document 2 (JP-A-2002-122782). The lens described in Patent Document 2 is a varifocal lens, so telecentricity is not maintained constant during power-varying and operation adjustment becomes complicated when power-varying is necessary after installation.
In consideration of such situations, the applicant has proposed a projection zoom lens having a proper back focal length, constant Fno. during zooming, a high zoom ratio and high compatibility, and has submitted to the Japanese Patent Office (see Patent Document 3 (JP-A-2009-128683 corresponding to US-A-2009/0135496) and Patent Document 4 (JP-A-2009-128684 corresponding to US-A-2009/0135497)).
The projection zoom lens according to the invention described in Patent Document 3 (hereinafter, referred to as “Proposed Invention 3”) is of a five-group configuration and three-group movement. In this projection zoom lens, an aperture adapted to be fixed during power-varying is arranged between the fourth lens group and the fifth lens group.
The projection zoom lens according to the invention described in Patent Document 4 (hereinafter, referred to as “Proposed Invention 4”) is of a six-group configuration and four-group movement. In this projection zoom lens, an aperture that is independently moved during power-varying is arranged between the fourth lens group and the fifth lens group.
The zoom lens of Proposed Invention 4can further increase an angle of view, as compared with the zoom lens of Proposed Invention 3. However, in the zoom lens of Proposed Invention 4, moving groups are arranged on the reduction side. Accordingly, by comparison of the outer diameter of a lens including a mechanism, the outer diameter of a lens on the reduction side in the zoom lens of Proposed Invention 4 becomes larger.
In general, the outer-diameter portion of a lens includes a fastening mechanism for fastening the lens to the projection-type display device. However, when a lens is fastened to an existing cinema-oriented projection-type display device, in many cases, the fastening mechanism is provided on the reduction side from the position of an aperture diaphragm (in the zoom lens of Proposed Invention 4, the position of an aperture diaphragm at a WIDE end).
In an existing projection-type display device, the size of a space for arranging a projection lens is set in advance. Accordingly, in order to mount a proposed new projection lens on an existing device, the outer diameter of a lens on the reduction side from a position where the fastening mechanism is arranged needs to have such a size as to be accommodated in the space for lens arrangement of the device.
However, the zoom lens of Proposed Invention 4 is configured such that the outer diameter of a lens on the reduction side from a position where the fastening mechanism is arranged becomes larger, as compared with the zoom lens of Proposed Invention 3. For this reason, the zoom lens may not be attached to an existing projection-type display device to which the zoom lens of Proposed Invention 3 can be attached.